Add guard for nil merge_request_diff.id

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -98,7 +98,7 @@ class Projects::MergeRequests::DiffsController < Projects::MergeRequests::Applic
         @merge_request.merge_request_diff
       end
 
-    return unless @merge_request_diff
+    return unless @merge_request_diff&.id
 
     @comparable_diffs = @merge_request_diffs.select { |diff| diff.id < @merge_request_diff.id }
 

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -16,6 +16,18 @@ describe Projects::MergeRequests::DiffsController do
         expect(response).to have_gitlab_http_status(:not_found)
       end
     end
+
+    context 'when the merge_request_diff.id is blank' do
+      it 'returns 404' do
+        allow_next_instance_of(MergeRequest) do |instance|
+          allow(instance).to receive(:merge_request_diff).and_return(MergeRequestDiff.new(merge_request_id: instance.id))
+
+          go
+
+          expect(response).to have_gitlab_http_status(:not_found)
+        end
+      end
+    end
   end
 
   shared_examples 'forked project with submodules' do